Persistent phosphorescence was discovered in the 11th century in China and Japan and in the 16th century in Europe (Shionoya, S. (1998) in Phosphor Handbook, Shionoya, S; and Yen, W. M; (eds.), CRC Press, Inc., Boca Raton, N.Y., p. 3, which is incorporated herein by reference for the corresponding discussion). The phenomenon involves two kinds of active centers, emitters and traps. Emitters are centers capable of emitting radiation after excitation of the center. Traps do not emit radiation, but store excitation energy and release it gradually to the emitter. Emitter centers can be created through the addition of activators, i.e., small amounts of intentionally added impurity atoms or ions, to the host matrix. Co-activators are additional, intentionally-added impurity ions which may affect (improve or modify) the emission of an activator. For example, a co-activator can be added to form trapping centers that can increase the persistence time of the phosphor.
Persistent phosphors are a class of luminescent materials that show long-lasting bright afterglow in dark after charged by UV light or another source of radiation capable of promoting transition of the material from one electronic state (e.g., the ground state) to an electronic state of higher energy (an excited state). Typically, long-persistent phosphors continue to emit an afterglow several hours after excitation has ceased. Various persistent phosphors in the visible have been developed. These include (a) sulfides, such as ZnS:Cu (green), ZnS:Cu, Co (green) and ZnS:Mn, Cu (green), CaS:Bi (460 nm, blue) and CaS:Eu,Tm (450 nm, red) developed before the sixties; (b) aluminates, such as CaAl2O4:Eu2+,Nd3+ (F. C. Palilla, A. K. Levine and M. R. Tomkus, Fluorescent Properties of Alkaline Earth Aluminate of the Type M2Al2O4 Activated by Divalent Europium, J. Electrochem. Soc. 115 (6) (1968) 642-644, which is herein incorporated by reference for the corresponding discussion), SrAl4O7:Eu2+,Pr3+/Dy3+ (W. M. Yen, W. Jia, L. Lu, H. Yuan, U.S. Pat. No. 6,117,362, 2000), SrAl2O4:Eu2+,Dy3+ (W. M. Yen, W. Jia, L. Lu and H. Yuan, U.S. Pat. No. 6,267,911 B1, 2001; H. Lange, U.S. Pat. No. 3,294,699, 12/1996; Murayama et al. U.S. Pat. No. 5,424,006; T. Matsuzawa, Y. Aoki, N. Takeuchi and Y. Murayama, A New Long Persistent Phosphor with High Brightness SrAl2O4:Eu2+,Dy3+, J. Electrochem. Soc. 143 (8) (1996) 2670-2673, which are herein incorporated by reference for the corresponding discussions); those phosphors have emission in the violet (440 nm), green-blue (490 nm) and green (520 nm), respectively; and (c) silicates, such as Sr2MgSi2O7:Eu2+,Dy3+ with phosphorescence at 470 nm (Zhiguo Xiao and Zhiqiang Xiao, U.S. Pat. No. 6,093,346, 2000), SrMgSi2O6:Eu2+,Nd3+ (470 nm) (D. Jia, W. Jia, Y. Jia, J. Appl. Phys. 101 (2007) Article Number: 023520, which is herein incorporated by reference for the corresponding discussion), and Ca3MgSi2O8:Eu2+,Dy3+ with afterglow band at 475 nm (Yuanhua Lin, Zhongtai Zhang, Zilong Tang, Xiaoxin Wang, Junying Zhang, Zhishan Zheng, J. Eur. Ceram. Soc. 21 (2001) 683). From the above list, it can be seen that all the emission of the persistent phosphors developed up to now are in the visible. The longest wavelength is in red near 650 nm (CaS:Eu2+,Tm3+). These visible persistent phosphors have been widely used for security signs, indicators of control panels, and so on.
It has been reported that some lanthanum gallates showed strong emission in the infrared and the single crystals were used for tunable infrared lasers. For example, La3Ga5SiO14:Cr3+ (A. A. Kaminskii, A. P. Shkadarevich, B. V. Mill, V. G. Koptev and A. A. Demidovich, Wide-band Tunable Stimulated Emission from a La3Ga5SiO14:Cr3+ Crystal, Inorg. Mater. (USSR) 23 (1987) 618; S. T. Lai, B. H. T. Chai, M. Long and M. D. Shinn, Room Temperature Near-Infrared Tunable Cr:La3Ga5SiO14 Laser, IEEE J. Quantum Electron. Qe-23 (1987) 24, which are herein incorporated by reference for the corresponding discussions); La3Ga5GeO14:Cr3+ (A. A. Kaminskii, A. P. Shkadarevich, B. V. Mill, V. G. Koptev, A. V. Butashin, and A. A. Demidovich, Tunable Stimulated Emission of Cr3+ Ions and Generation Frequency Self-Multiplication Effect in Acentric Crystals of La-gallogermanate Structure, Inorg. Mater. (USSR), 24 (1988) 579, which is herein incorporated by reference for the corresponding discussion); La3Ga5.5Nb0.5O14:Cr and La3Ga5.5Ta0.5O14:Cr (A. A. Kaminskii, A. P. Shkadarevich, B. V. Mill, V. G. Koptev, A. V. Butashin and A. A. Demidovich, Wide-band Tunable Stimulated Emission of Cr3+ Ions in the Trigonal Crystal La3Ga5.5Nb0.5O14, Inorg. Mater. (USSR) 23 (1987) 1700, which is herein incorporated by reference for the corresponding discussion). However, no afterglow has been reported in the luminescent crystals.